1. Field of the Invention
The present invention relates to alkyl substituted resorcinolic novolak resins which are used with curing agents in rubber to give improved mechanical properties and tire cord to rubber adhesion.
2. Brief Description of the Prior Art
Resorcinol and resorcinol-formaldehyde resins have been used in the rubber industry as reinforcing and bonding agents in the rubber compounds. These resorcinolic compounds and resins are unique materials for rubber compounding since they act as thermosetting or vulcanizing plasticizers. They are very efficient plasticizers for rubber during the processing operations. This allows easier processing, higher loading and excellent extrusions for the rubber compounds.
The thermosetting properties of resorcinol and resorcinol based resins on curing allow the vulcanizate to show increase in hardness, abrasion resistance, aging resistance, solvent and oil resistance and stiffness, and aldo give much improved finishes to the cured rubber stock. This combination of plasticizing and reinforcing action is rare for a single material. In addition, these resorcinolic resins may act as antioxidants in the rubber compound.
Though resorcinol and resorcinol-formaldehyde resins provide good mechanical and adhesion properties, fuming and hygroscopicity of these compounds present problems in rubber compounding and handling. The hygroscopic behavior of resorcinol and resorcinol-formaldehyde resins can be reduced by the inclusion of a hydrophobic aliphatic hydrocarbon molecule. This can be achieved by alkylating resorcinol or resorcinol-formadehyde resins with unsaturated hydrocarbons in the presence of an acid catalyst. The volatility of the resorcinol derivative is lower than that of resorcinol due to the presence of alkyl substituents.
The properties of rubber compounds containing resins also depend on their miscibility. Significant miscibility is seen with alkylphenol-formaldehyde resins in general purpose rubbers. Therefore, a significant increase in the tack of rubber compounds based on isoprene, butadiene and styrene-butadiene is expected to take place with alkylated resorcinolic resins in which the alkyl group contains at least four carbon atoms.
The alkylation reaction of phenols can be carried out with an olefin or alcohol using Friedel-Crafts or an acid catalyst. The alkylated phenols have been used extensively as antioxidants in the rubber compound formulation. Similarly, alkyl substituted resorcinolic compounds have been successfully prepared using tert-butyl alcohol, isoamylene, diisobutylene in the presence of an acid catalyst.
Alkyl substituted resorcinolic novalak type resins could be made without isolating the alkyl resorcinol. This can be achieved by first reacting resorcinol with an unsaturated hydrocarbon, like piperylene, dicyclopentadiene, dipentene and divinyl benzene using an acid catalyst. Then an aldehyde such as formaldehyde can be added into the above reaction mixture to produce the novalak type resins.
It is, therefore, an object of the present invention to provide alkyl substituted novalak resins that are capable of crosslinking with curing agents in the rubber to give improved mechanical and cord to rubber adhesion properties.